The potential safety hazards and limited lithium resources of lithium‐ion batteries (LIBs) have restricted their practical application. Potassium‐ion batteries (KIBs) are a novel energy storage technology with great cost advantages and are a promising alternative to LIBs. However, because of the large ionic radius of K+, the common anode materials used in LIBs exhibit a large volumetric expansion and structural collapse in the process of charging and discharging in a KIB. In this work, the prepared nitrogen‐doped porous carbon fibers (P‐CFs) from bombyx mori silk cocoons via a simple KCl activation and high‐temperature carbonization method. The P‐CFs possess a large specific surface area and one‐dimensional porous structure, offering facile storage and fast transport channels for K+ ions. When used as an anode for KIBs, the P‐CFs exhibit a high reversible specific capacity of 275 mAh·g−1 at a current density of 0.1 A·g−1 after 500 cycles and even retain a capacity of 100 mAh·g−1 after 5000 cycles at 2 A·g−1. This study demonstrates a simple strategy for the low‐cost synthesis of nitrogen‐doped porous carbon fibers from an environmental friendly biomaterial, with excellent electrochemical performance as an anode for KIBs.